Nine times out of ten the mix is wrong, not the machine. The Ninja Slushi needs at least 4% sugar by volume to form slush, and no more than 16% alcohol. Diet soda, plain water, unsweetened coffee, and heavy pours of liquor all sit outside that window, so the machine churns liquid forever. Fix the ratio and the same machine freezes fine.
Sugar is what lets the mix form soft slush instead of solid ice or cold liquid. The machine's hard minimum is 4% sugar by volume, and 8 to 15% is the sweet spot for texture. Most real juices already qualify: orange juice is around 10%, apple and cranberry around 12%. Zero-sugar drinks do not qualify, no matter how sweet they taste, because artificial sweeteners do not lower the freeze point.
Alcohol lowers the freezing point. Past 16% ABV in the final mix, the machine cannot get cold enough. For spirits like vodka or tequila, the working guideline is about 4 oz of liquor per 24 oz of total mix. If you poured half a bottle of vodka into a pitcher of lemonade, that is why it never set.
Bottled margarita and cocktail mixes are formulated for ice or blenders, not slush machines. Many are too concentrated, and adding the liquor the label suggests pushes the mix past the alcohol ceiling. Cut them with water or juice, and go lighter on the pour than the bottle says.
A normal batch takes 15 to 30 minutes. High-sugar (15%+) or boozier mixes (12%+ ABV) can take 45 to 75 minutes. If the consistency is close but loose, let it keep running before changing anything.
Check the vessel is seated, the room is not hot, and the machine had time to pre-chill. If a known-good recipe (say, straight orange juice) also refuses to freeze, then it actually is the machine, and that is warranty territory.
Almost always a ratio problem: under 4% sugar or over 16% alcohol in the final mix. Add sugar (or cut the alcohol with juice) and it will set.
Not on its own. Artificial sweeteners like stevia and erythritol do not lower the freeze point. Add allulose (about half a tablespoon per 8 oz) or blend in some regular sugar.
15 to 30 minutes for a standard mix. High-sugar or high-alcohol mixes can take 45 to 75 minutes.